Jointing material and joining method

ABSTRACT

A coating film ( 102 ) is formed on a substrate ( 101 ) of sapphire by coating the substrate with a jointing material prepared by dissolving Al(OC 2 H 5 ) 3  and B(OCH 3 ) 3  into xylene. Subsequently, the substrate ( 101 ) is heated to about 450° C. and the coating film ( 102 ) is irradiated with, e.g., ultraviolet radiation having a wavelength of 172 nm. Thus, xylene in the coating film ( 102 ) is vaporized, and Al(OC 2 H 5 ) 3  and B(OCH 3 ) 3  are thermally decomposed to form a jointing material composed of amorphous or γ-layer alumina (Al 2 O 3 ) and amorphous boron oxide (B 2 O 3 ).

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a jointing material made of ametal oxide and, more particularly, to a jointing material and joiningmethod of joining a metal oxide such as sapphire with another member.

[0002] A capacitive pressure sensor chip is available which detects apressure by detecting an electrostatic capacitance. The housing of thepressure sensor chip is constituted by a base body having apredetermined space and a diaphragm arranged on the space of the basebody. The pressure sensor chip has a stationary electrode arranged onthe base body and a movable electrode fixed to the diaphragm. When thediaphragm deforms upon reception of a pressure, the distance between themovable electrode and stationary electrode changes, so the electrostaticcapacitance between the movable and stationary electrodes changes. Thepressure received by the diaphragm can be measured on the basis of thechange in electrostatic capacitance.

[0003] As such a pressure sensor chip, one in which the base body anddiaphragm which constitute the housing are made of sapphire is proposed.Sapphire, i.e., corundum (aluminum oxide, a phase), is thermally verystable, hardly dissolves in an acid or alkali, and has wide applicationssuch as in a refractory, insulator, and abrasive. When sapphire havingsuch properties is used to form the housing, even if the measurementtarget is a corrosive fluid, the pressure of the fluid can be measuredby receiving the fluid with the diaphragm directly.

[0004] The above pressure sensor chip is fixed to a base as shown inFIG. 5, and is used as a pressure sensor. As shown in FIG. 5, a chip 501in the pressure sensor is placed in a recess 503 formed at the center ofa base 502 made of, e.g., glass, such that its diaphragm portion facesup in FIG. 5. Terminals 505 of electrode pins 504 extending through thebottom surface of the base 502 are present in the bottom surface of therecess 503 of the base 502, and are connected to the interconnections ofthe respective electrodes extended to the rear surface of the base ofthe chip 501. A vent hole 506 communicating with the bottom surface ofbase 502 is formed in the bottom surface of the recess 503.

[0005] The chip 501 is pressed at the peripheral portion of its uppersurface by a cover plate 507 having an opening at its center, and isfixed to the recess 503. The cover plate 507 is joined to the uppersurface of the base 502 with glass which is fused once, so that it isfixed to the base 502. The abutting portions of the cover plate 507 andchip 501 are connected hermetically so the measurement target fluidwhich is to come into contact with the upper surface of the chip 501does not enter the recess 503 around the chip 501.

[0006] The cover plate 507 is made of sapphire which is the samematerial as that of the chip 501, in order that the precision ofpressure measurement is secured and that deformation of the chip 501 bythe temperature becomes identical, thus causing no stress in the chip501. The abutting surfaces of the cover plate 507 and chip 501 arejoined by direct joining in order to eliminate hetero-joining, so thatthe hermetic state described above can be obtained. In this sapphirejoining, the two joining surfaces are mirror-polished and abuttedagainst each other. Pressure is applied across the two sapphire membersto be joined, and the sapphire members are heated. Thus, a firm joiningstate can be obtained without using any jointing material or the like.

[0007] In above direct joining, as the abutting surfaces of the twosapphire members must be mirror-polished until their surface roughnessesbecome 0.3 nm or less, the member (cover plate) becomes very expensive.If a generally available jointing material or the like is used, thejoining surfaces need not be mirror-polished. However, as a differentmaterial is present between the joining surfaces, a stress may occur, orthe joining portions cannot have corrosion resistance or thermalstability equivalent to those of sapphire. Then, the applications arelimited.

[0008] As described above, conventionally, when a component is to beformed by joining two or more members made of a metal oxide such assapphire, to exhibit 100% the properties of the material that forms themembers, the members are joined to each other by direct joining.However, direct joining is costly. In view of this, when the members areto be joined to each other by utilizing a conventionally availablejointing material, a component can be formed at a low cost, but theproperties of the material that forms the members cannot be utilized100%.

SUMMARY OF THE INVENTION

[0009] The present invention has been made to solve the problemsdescribed above, and has as its object to enable members made of a metaloxide such as sapphire to be joined to each other at a low cost withoutimpairing the characteristics of the members.

[0010] A jointing material according to one aspect of the presentinvention comprises a mixture of boron oxide and intermediate-phasealuminum oxide which is more unstable than a-phase aluminum oxide andwhich tends to change to a lower energy level.

[0011] According to this jointing material, when it is heated to amelting point or more of boron oxide, it forms a mixture of aluminumoxide and Al₁₈B₄O₃₃.

[0012] A jointing material according to another aspect of the presentinvention comprises aluminum oxide and Al₁₈B₄O₃₃.

[0013] According to this jointing material, it has a heat resistance of1,000° C. or more.

[0014] A joining method according to one aspect of the present inventioncomprises the first step of forming, on a joining surface of a base bodyof a metal oxide, a solution layer formed of a solution in which a firstcompound containing aluminum and oxygen and a second compound containingboron and oxide dissolve, the second step of heating the solution layerso that the first compound forms intermediate-phase aluminum oxide whichis more unstable than a-phase aluminum oxide and which tends to changeto a lower energy level, that the second compound forms boron oxide, andthat a joining layer formed of a jointing material containingintermediate-phase aluminum oxide and boron oxide is formed on thejoining surface of the base body, and the third step of arranginganother member on the joining layer and heating the joining layer to notless than a melting point of boron oxide for a predetermined period oftime.

[0015] According to this joining method, the jointing material of thejoining layer forms a mixture of aluminum oxide and Al₁₈B₄O₃₃ throughthe third step, thus joining the base body and another member.

[0016] In the above joining method, the metal oxide is α-phase aluminumoxide. It suffices if the first compound is an organic metal compound ofaluminum, and the second compound is an organic metal compound of boronsuch as an alkoxide. Alternatively, the first compound can be analuminum hydroxide such as boehmite, and the second compound can beboric acid. Intermediate-phase aluminum oxide may be any one or amixture of γ phase, θ phase, ι phase, κ phase, ε phase, χ phase, δphase, and σ phase, or in an amorphous state.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a view showing a step for explaining a joining methodaccording to an embodiment of the present invention;

[0018]FIG. 2 is a view showing a step which follows FIG. 1 to explainthe joining method;

[0019]FIG. 3 is a view showing a step which follows FIG. 2 to explainthe joining method;

[0020]FIG. 4 is a view showing a step which follows FIG. 3 to explainthe joining method; and

[0021]FIG. 5 is a view showing the arrangement of a pressure sensor.

DETAILED DESCRIPTION OF THE EMBODIMENT

[0022] The embodiment of the present invention will be described withreference to the drawings.

[0023] <First Embodiment>

[0024]FIG. 1 is a view showing a step for explaining a joining methodaccording to an embodiment of the present invention. This embodimentwill be explained by way of a case in which sapphire substrates are tobe joined to each other.

[0025] First, as shown in FIG. 1, a coating film 102 is formed on asubstrate 101 of sapphire by coating the substrate with a jointingmaterial prepared by dissolving Al(OC₂H₅)₃ and B(OCH₃)₃ into xylene.Subsequently, the substrate 101 is heated to about 450° C. and thecoating film 102 is irradiated with, e.g., ultraviolet radiation havinga wavelength of 172 nm. Thus, xylene in the coating film 102 isvaporized, and Al(OC₂H₅)₃ and B(OCH₃)₃ are thermally decomposed.

[0026] Hence, a joining layer 103 made of a jointing material composedof amorphous or γ-layer alumina (Al₂O₃) and amorphous boron oxide (B₂O₃)is formed on the substrate 101 (FIG. 2). The heating temperature islower than 470° C., which is the melting point of boron oxide.

[0027] The surface of the joining layer 103 formed by coating with thesolution described above is flat as it absorbs steps on the surface ofthe substrate 101. Part of alumina in the joining layer 103 formed bythermal decomposition is joined to the surface of the substrate 101 ofsapphire.

[0028] As alumina has a corundum type crystal structure (α phase), itforms sapphire or the like which is very stable as corundum. In contrastto this, γ-alumina having a cubic system spinel type crystal structurehas may lattice defects in which oxygen atoms lack because, e.g., thecrystal structure is unnatural. In amorphous alumina, many latticedefects exist. When a layer having many lattice defects in which oxygenatoms lack is formed on a metal oxide crystal body such as sapphire inthis manner, oxygen on the surface enters the lattice defects to try tocreate a stable state. Thus, the substrate 101 and the joining layer 103formed on its surface are chemically bonded and joined to each other.

[0029] A new substrate 104 of sapphire is prepared as shown in FIG. 4.The substrate 104 is placed on the joining layer 103 on the substrate101. The resultant structure is heated to 470° C. or more, e.g., 480° C.This heating temperature should be equal to or higher than a temperatureat which boron oxide fuses. At about 480° C., an equilibrium state isobtained where γ-alumina does not fuse at all but partly dissolves inboron oxide which has dissolved to form a liquid. In other words, in thejoining layer 103, γ-alumina partly dissolves in liquefied boron oxide.

[0030] In this state, boron oxide which has fused to form liquid andγ-alumina which dissolves in it react with each other to form Al₂B₂O₆.

[0031] At the heating state at 480° C., as the temperature is equal toor higher than the melting point of Al₂B₂O₆, Al₂B₂O₆ fuses to form aliquid. Then, remaining γ-alumina partly dissolves in and reacts withfused Al₂B₂O₆, thus forming Al₄B₄O₉. As Al₄B₄O₉ fuses at 480° C.,remaining γ-alumina successively partly dissolves in and reacts withmolten Al₄B₄O₉ liquid to form Al₁₈B₄O₃₃ finally. As Al₁₈B₄O₃₃ has amelting point of 1,950° C., it does not dissolve when it is heated to480° C., but forms a solid.

[0032] Therefore, when the joining layer 103 is held heated at 480° C.,its boron oxide portion fuses in the initial stage. Fused boron oxideand alumina chemically react with each other to form Al₁₈B₄O₃₃ having amelting point of 1,950° C. As a result, as shown in FIG. 4, thesubstrate 101 and substrate 104 are firmly joined to each other througha joining layer 103 a of alumina and Al₈B₄O₃₃. The joining layer 103 ais thermally very stable, and hardly dissolves in an acid or alkali,which are characteristics similar to those of sapphire.

[0033] In the above description, Al(OC₂H₅)₃ and B(OCH₃)₃ are used, butthe present invention is not limited to them. For example, when analuminum hydroxide such as boehmite sol and boric acid are used, if acoating film formed by dissolving them is heated, a joining layer 103made of a jointing material composed of amorphous or γ-layer alumina(Al₂O₃) and amorphous boron oxide (B₂O₃) can be formed on the substrate101.

[0034] As has been described above, according to the present invention,a plate member of sapphire can be joined to a pressure sensor chip ofsapphire in the same state as direct joining. An excellent effect canthus be obtained in which a pressure sensor made of sapphire can beformed at a low cost without impairing the properties of sapphire.

[0035] As described above, the jointing material and joining methodaccording to the present invention are suitable when members made of ametal oxide such as sapphire are to be joined to each other at a lowcost without impairing the characteristics of the members.

1. A jointing material characterized by comprising a mixture of boronoxide and intermediate-phase aluminum oxide which is more unstable thanα-phase aluminum oxide and which tends to change to a lower energylevel.
 2. A jointing material characterized by comprising aluminum oxideand Al₁₈B₄O₃₃.
 3. A joining method characterized by comprising the firststep of forming, on a joining surface of a base body of a metal oxide, asolution layer formed of a solution in which a first compound containingaluminum and oxygen and a second compound containing boron and oxidedissolve, the second step of heating the solution layer so that thefirst compound forms intermediate-phase aluminum oxide which is moreunstable than a-phase aluminum oxide and which tends to change to alower energy level, that the second compound forms boron oxide, and thata joining layer formed of a jointing material containingintermediate-phase aluminum oxide and boron oxide is formed on thejoining surface of the base body, and the third step of arranginganother member on the joining layer and heating the joining layer to notless than a melting point of boron oxide for a predetermined period oftime.
 4. A joining method according to claim 3, characterized in thatthe metal oxide is α-phase aluminum oxide.
 5. A joining method accordingto claim 3, characterized in that the first compound is an organic metalcompound of aluminum and the second compound is an organic metalcompound of boron.
 6. A joining method according to claim 5,characterized in that the second compound is boron alkoxide.
 7. Ajoining method according to claim 3, characterized in that the firstcompound is an aluminum hydroxide, and the second compound is boricacid.
 8. A joining method according to claim 7, characterized in thatthe first compound comprises boehmite.
 9. A joining method according toclaim 3, characterized in that intermediate-phase aluminum oxide is anyone or a mixture of γ phase, θ phase, ι phase, κ phase, ε phase, χphase, δ phase, and σ phase, or in an amorphous state.